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EEEV [Eastern Equine Encephalitis Virus] in Mosquitoes | CDC EID
EID Journal Home > Volume 16, Number 12–December 2010
Volume 16, Number 12–December 2010
Eastern Equine Encephalitis Virus in Mosquitoes and Their Role as Bridge Vectors
Philip M. Armstrong Comments to Author and Theodore G. Andreadis
Author affiliation: The Connecticut Agricultural Experiment Station, New Haven, Connecticut, USA
Suggested citation for this article
Eastern equine encephalitis virus (EEEV) is maintained in an enzootic cycle involving Culiseta melanura mosquitoes and avian hosts. Other mosquito species that feed opportunistically on mammals have been incriminated as bridge vectors to humans and horses. To evaluate the capacity of these mosquitoes to acquire, replicate, and potentially transmit EEEV, we estimated the infection prevalence and virus titers in mosquitoes collected in Connecticut, USA, by cell culture, plaque titration, and quantitative reverse transcription–PCR. Cs. melanura mosquitoes were the predominant source of EEEV (83 [68%] of 122 virus isolations) and the only species to support consistently high virus titers required for efficient transmission. Our findings suggest that Cs. melanura mosquitoes are primary enzootic and epidemic vectors of EEEV in this region, which may explain the relative paucity of human cases. This study emphasizes the need for evaluating virus titers from field-collected mosquitoes to help assess their role as vectors.
During the past 6 years, eastern equine encephalitis virus (EEEV; family Togaviridae, genus Alphavirus) has reemerged in the northeastern United States and resulted in 26 human cases of infection and 9 deaths (ArboNET; Centers for Disease Control and Prevention, Atlanta, GA, USA). Virus transmission has intensified throughout this region, spread to locales where it had not been previously detected, and extended north into New Hampshire and Maine, USA, and Nova Scotia, Canada (1). Disease outbreaks caused by EEEV occur at irregular intervals when underlying ecologic conditions favor virus amplification and overflow into human and equine populations.
EEEV is perpetuated in an enzootic cycle involving ornithophilic mosquitoes (primarily Culiseta melanura) and passerine birds in freshwater swamps (2,3). Human and equine cases occur infrequently despite relatively high rates of EEEV infection in Cs. melanura during virus amplification (ArboNET). Other mosquito species such as Aedes vexans, Coquillettidia perturbans, Ochlerotatus canadensis, and Oc. sollicitans have been implicated as epidemic/epizootic bridge vectors from viremic birds to horses and humans (4–6). These species are competent vectors of EEEV (7–9) and may acquire virus infection during disease outbreaks by feeding occasionally on birds but prefer mammalian hosts (10–14). Although Cs. melanura mosquitoes feed infrequently on mammals (10,12,15), their ability to serve as a bridge vector may be offset by a much higher prevalence of EEEV infection in this species.
One criterion used for incriminating enzootic and bridge vectors is based on the frequency of virus detection from each candidate species (16). Typically, mosquitoes are collected from disease-endemic sites, sorted into pools by trap location and species, and screened for virus by cell culture or molecular methods. This procedure provides critical information on the identity, spatial and temporal distribution, and proportion of virus-infected mosquitoes and forms the basis for many arbovirus surveillance programs. Nevertheless, virus titers may vary considerably within infected mosquitoes (17) and reflect the duration of the extrinsic incubation period and the ability of these mosquitoes to support virus replication, which is a necessary precondition for mosquitoes becoming infectious (18). The virus must undergo several rounds of replication in the mosquito midgut and salivary glands before being biologically transmitted to the vertebrate host when the mosquito salivates during blood feeding.
In Connecticut in 2009, EEEV activity increased substantially, and we isolated numerous viruses from Cs. melanura mosquitoes and potential bridge vectors. To evaluate the capacity of these mosquitoes to replicate and potentially transmit virus, we estimated EEEV titers from virus-positive mosquito pools with the expectation that the most efficient vectors will support consistently high virus titers.
EEEV in Mosquitoes | CDC EID
Suggested Citation for this Article
Armstrong PM, Andreadis TG. Eastern equine encephalitis virus in mosquitoes and their role as bridge vectors. Emerg Infect Dis [serial on the Internet]. 2010 Dec [date cited]. http://www.cdc.gov/EID/content/16/12/1869.htm
Comments to the Authors
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Philip M. Armstrong, Center for Vector Biology and Zoonotic Diseases, Department of Environmental Sciences, The Connecticut Agricultural Experiment Station, 123 Huntington St, PO Box 1106, New Haven, CT 06504, USA; email: firstname.lastname@example.org